Every gemstone has it's own Specific Weight, for example a Sapphire is 4, and Amethyst is 2.65. Thats mean Sapphire always are heavier than the same size Amethyst.

Typically you can expect an average weight around 0.20 Grs per carat.

Depending on the cut of the gemstone it can look longer or shorter, Marquise cut Topaz for example, 12 x 6 mm, measure around 1.88 carats. The same Topaz gemstone with Square Cushion Cut, 8 x 8 mm, can look smaller and be actually bigger on carats, around 2.70 Cts. Despite the fact of it's appearance, Cushion/Round/Square are typically deeper than a Marquise cut.

Another important fact about cut: Calibrated gemstones are allways well proportioned. For example a Pear Cut Calibrated have 12x8 mm, 14x10 mm, 16x12 mm, always follow the same proportions.